Mixing gas and filling tanks can be dangerous, in the same way as diving can be dangerous.

This program is not for novice divers, you must have training in mixing gas and understand the theory behind this practice; training for Mixed Gas Blending is available from various SCUBA Diving training agencies.
 

This program is a calculations software and does not claim to be a guide for handling blending risk, nevertheless it's worth taking a moment of your time to reflect on this issue.

There are several levels of danger about filling:

INAPPROPRIATE MIXTURES

It is worth remembering that you have to be aware that low or excessive levels of Oxygen can be fatal, and also how an incorrect composition of Nitrogen/Helium can be extremely dangerous.
You need to know all the implications on gas mixing and dive planning: there is no substitute for  good training! It is your responsibility to check the software output and to analyze any gas that you mix. You have to double-check the software output, the partial pressures and the analysis readings; Oxygen and Helium analyzers must be used to measure the content of the mix.

HIGH PRESSURE

Handling high pressure equipment and gases can be extremely dangerous if done without adequate knowledge.
The cylinders must be tested according to national legislation and manufacturer's recommendations.
Pay attention to improperly assembled parts, like threads, hoses etc.
Do not attempt to handle high pressure oxygen or fill SCUBA cylinders without proper training.

COMBUSTION: OXYGEN FIRE

The use and  handling of high pressure Oxygen can be extremely dangerous if done without knowledge; you have to be aware that it can cause fires or explosion if handled improperly.
Oxygen is not a fuel or explosive by itself, but it can decrease the ignition temperature of the materials and increase the combustion rate. To cause a fire or explosion, it is necessary to have three things: fuel, an oxidizer and heat; this is known  as the "Fire triangle" or "Combustion triangle".

In Oxygen filling all the system components must be Oxygen clean and Oxygen compatible.

For more details see  Safety precautions - A  more advanced step